Cross-linking polysilanes

By comparison with the reactions as shown in Eqs. (15) and (16), dehydrocoupling (without redistribution) of 4 catalyzed by Cp2MCl2/2n-BuLi gives mostly highly cross-linked polysilanes along with soluble oily polymers as minor products [Eq. (17)].56 58... 

The lutetium hydride complex [Cp 2Lu(/t-H)]2 efficiently cleaves the Si-C bond of PhSiH3 to produce benzene and cross-linked polysilanes (SiHJ (Scheme 290).604 Formation of Ph2SiH2 and SiH4 has also been observed during the samarium-catalyzed redistribution of PhSiHj.545... 

Functionalized polysilanes, such as those bearing reactive alkenyl, halo-, amino-, or alkoxy substituents, are chemically reactive, permitting cross-linking, or further functionalization reactions, as already described in Section 3.11.4.2. Flowever, the Si-Si bond can itself react chemcially under a variety of conditions. 

Sanji T, Nakatsuka Y, Ohnishi S, Sakurai H. Preparation of nanometer-sized hollow particles by photochemical degradation of polysilane shell cross-linked micelles and reversible encapsulation of guest molecules. Macromolecules 2000 33 8524-8526. 

When aryl substituent groups are present, more complex photolytic reactions occur, and some cross-linking takes place as well as chain scission. Groups containing C=C double bonds are even more effective at bringing about cross-linking. This leads directly to the topic of cross-linking in polysilanes. 

For some uses it is important to form bonds that link different polysilane chains, to transform soluble, meltable polysilanes into insoluble resins. This process is vital if the polysilanes are to be used as precursors to silicon carbide ceramics, since, if cross-linking is not carried out, most of the polymer is volatilized before thermolysis to silicon carbide can take place. Several methods have therefore been developed to bring about cross-linking of polysilanes.109 110... 

Completely networked polysilanes show a variation of n and k with the degree of cross-linking compared to parent linear polysilanes. 

The polysilane core part within the shell cross-linked micelles was photodegraded by UV irradiation (>280nm) and dialysis against water produced nanometer-sized... 

The anionic polymerization of masked disilenes proceeds via living anions, and therefore block copolymerization with a conventional vinyl monomer is possible. Recently, interesting hydrophobic block copolymer of PMHS with poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(methacrylic acid) (PMMA) have been prepared (Scheme 11). These polymers can be self-assembled and are transformed into polysilane micelles, shell cross-linked micelles (SCM), and nanometer-sized hollow particles. ... 

In these equations, and are the initial molecular weights, 4>(s) and 4>(x) are the quantum yields for the scissioning and cross-linking reactions, respectively D is absorbed dose and is Avogadro s number. The slopes of the respective plots of I/M and I/M versus dose produce two simultaneous equations, the solution of which yields values for 4>(s) and 4>(x). We (60) have analyzed the data obtained for a number of polysilane derivatives by GPC (gel permeation chromatography) to evaluate the respective molecular weights and distributions (Table III). Polystyrene standards were used for molecular weight calibration. 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

In addition, mechanistic studies of the photochemical reactions are necessary to determine whether similar processes occur in the solid state. Polymer chain scission is usually the predominant process in the solid state, although cross-linking reactions become more important in the presence of pendant unsaturation. However, little is known about the nature of the intermediates produced in the solid state. Information of this type is important, because most of the applications of polysilane derivatives require the materials as solid films. 

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